1. Field of the Invention
The present invention relates to fume exhaust ducts and more particularly to a dual wall duct having a laminated inner wall whose innermost layer incorporates carbon fibers which dissipate and neutralize built-up electrostatic charges resulting from vapors or gases flowing within the duct.
2. Description of the Related Art
Ductwork for corrosive or otherwise dangerous vapor and gas exhaust systems is used extensively in many diverse industries which utilize hazardous chemicals to process raw materials or perform manufacturing procedures, such as the semiconductor industry, the plating industry, and the pharmaceutical industry. Such ductwork also is required in the many research and development laboratories which use highly reactive, toxic or otherwise hazardous chemicals in conducting experiments. Consequently, vapors from such chemicals must be exhausted through leak-proof air ducts to safely remove them from work areas. Installations can be very large, consisting of many thousands of feet of ductwork which may be manifolded and connected to multiple exhaust fans. Because of the wide diversity of chemicals used in industrial and research applications, it is extremely difficult to provide a single material for fabricating ductwork which can withstand all the chemicals to which duct interiors may be exposed. Materials which have been used heretofore to fabricate fume exhaust ductwork include black steel, galvanized steel and stainless steel, as well as plastic materials such as polyvinylchloride, polypropylene, coated materials, and fiberglass reinforced plastics (FRP's). Over the past forty years the trend in materials has been away from metals and coated metals and toward the use of plastics, particularly FRP's.
Various types of resins have been used in manufacturing FRP's. Some are resistant to certain families of chemicals, but no single resin can resist all the vapors used in industries such as semiconductor manufacturing and electroplating. Another problem is providing adequate resistance to fire. Unlike metallic ducts, plastic ducts exhausting chemicals which can react exothermically with themselves or with duct surfaces are at risk to combustion. Plastics typically burn rapidly and generate much smoke, creating hazards of their own. A third problem in using plastic is the possibility of static electrical charge building up on a duct's interior surface. Electrostatic charge is usually generated by frictional processes during contact and separation of materials. So-called "triboelectric charging" can occur even when dry media such as air or air laden with dust particles flow through ungrounded plastic pipes. Because plastic is an insulator, a considerable amount of charge can be generated on a duct's inner surface. An insulative material does not readily allow the flow of electrons, so both positive and negative charges can reside on the surface at the same time, although at different sites.
If the voltage at a site builds up sufficiently, an electrostatic discharge will occur creating an arc which, depending on the characteristics of the ambient vapor, can trigger an explosion or fire. Perhaps the worst cases when exhausting hazardous vapors are those containing halogens such as fluorine or chlorine and those containing metallic vapors such as cesium or potassium. Such vapors are easily ionized as well as highly corrosive and/or toxic.
U.S. Pat. Nos. 5,298,299 ("'299") and 5,383,994 ("'994") to L. E. Shea, which are incorporated in their entirety herein by reference, are directed to, respectively, a composite fume exhaust duct having both good chemical resistance and good fire resistance properties, and a method for making the dual-laminate duct. Ducts made in accordance with the invention described in these patents are generally tubular with a diametral size in the range of 2 inches to 84 inches, and have an inner laminate portion of chemically resistant material covered by and integral with an outer laminate portion of fire retardant material. The inner laminate is comprised of material such as fiberglass which is saturated with a chemically resistant resin such as a halogenated vinyl ester. The outer laminate which covers the inner laminate is similarly comprised of fabric or fiberglass material which is combined with a resorcinol or phenol/resorcinol type fire-retardant resin. The duct is formed by first coating a mylar wrapped mandrel with the chemically resistant resin and then wrapping the mandrel with successive layers of fabric material saturated with the resin. The outer fire-retardant laminate is then formed directly over the inner laminate by applying successive layers of a suitable fabric material saturated with the fire-retardant resin. The composite structure is then allowed to cure and harden before being removed from the mandrel. Such ducts do not, however, eliminate or even mitigate the hazard of static charge build-up on the innermost surface of the inner laminate, which typically has a resistivity of the order of 10.sup.14 -10.sup.15 ohms-cm.
Techniques for dissipating electrostatic charges in plastic pipe systems without the need to ground each pipe section individually are described in the related art. U.S. Pat. No. 4,120,325 to W. J. de Putter discloses a polyvinylchloride pipe having an outer and/or an inner surface coated with a layer of thermosetting epoxy material containing carbon particles so that the layer is electrically conductive. An elastomeric seal connecting two sections of pipe also contains carbon particles so multi-section piping need only have a single ground. U.S. Pat. No. 4,635,162 to H. T. McLaughlin discloses an electrically conductive flange for joining a conduit and fitting to each other while providing electrical connectivity from the conduit and fitting to bolts used to secure their mating flanges. The flange body and an annular rim are non-conductive and at least one strand of conductive yam is incorporated within a bore which receives the conduit end. Each strand is in conductive contact with the inside surface of the conduit and extends to the rim to contact a bolt. The flange provides for a continuous conductive pathway along a piping system, so grounding can take place at a single location.
Neither device provides a solution to eliminating the possibility of electrostatic discharge occurring when exhausting chemically reactive fumes. The duct inner surface must be able to withstand corrosion and other chemical reaction with the fumes while also neutralizing triboelectric charges. Ideally, there would be no need to electrically ground individual duct sections or even an entire ductwork assembly if its duct sections were electrically connected.